(a) Field of the Invention
The present invention relates to a method and apparatus that performs protection switching on networks.
(b) Description of the Related Art
As a fault occurs at a working path that connects two end point nodes, when traffic transmission is stopped, protection switching is a method of continuing to transmit traffic between the two end point nodes using another path, called a protection path. Protection switching methods are divided into a linear protection switching method, a ring protection switching method, and a mesh protection switching method according to topology of a network.
In linear protection switching, for traffic that is transmitted in a bi-direction or uni-direction between two end points, a working path and a protection path are set to be disjointed. In principle, while traffic is being transmitted through a working path, a fault occurs at the working path or when an instruction of an operator to switch the traffic to protection path is input, traffic is transmitted to the protection path.
Ring protection switching is necessary in a ring network that is physical or logically formed. When a fault occurs at a link or a node of the ring network or when an instruction of an operator is input, traffic is transmitted through a protection path. In this case, operation methods of protection switching are different according to transfer technology of traffic, for example, Ethernet, a multiprotocol label switching (MPLS), and a multiprotocol label switching-transport profile (MPLS-TP).
Mesh protection switching sets a linear protection domain for a point-to-point connection when multiple point-to-point connections exist on a mesh topology network and enables a protection path of a plurality of linear protection domains to share the same network resource.
In various forms of protection switching methods such as linear, ring, or mesh, when traffic is injected into a network, at both end nodes (linear protection switching and mesh protection switching) of a point-to-point connection or at a node belonging to a ring (ring protection switching), a transmission path of traffic is determined according to whether a fault exists (or an operator instruction is input) at a working path or a protection path that can transmit traffic. In this case, a method in which a node selects a transmission path of traffic or an apparatus that performs such a function is generally a referred to as a bridge.
A standard that is related to protection switching is generally defined through several standard recommendations in an international telecommunication union (hereinafter, referred to as an ‘ITU’), which is an international standard organization. An example of standard recommendations of the ITU is ITU-T G.870, G.808.1, G.873.1, G.8031, G.8032, G.8131, and G.8132, and a standardization organization such as Internet engineering task force (IETF) and the institute of electrical and electronics engineers (IEEE) has a plurality of protection switching related standards.
A bridge that is defined by the protection switching related standard includes a permanent bridge, a selector bridge, and a broadcast bridge.
The permanent bridge copies input traffic and always transmits the input traffic to a working path and a protection path regardless of a fault of a path. The selector bridge transmits traffic to a path having no fault according to whether a fault exists at a working path or a protection path. The broadcast bridge transmits traffic to only a working path, if a fault does not exist at the working path and copies traffic and transmits traffic to the working path and a protection path, if a fault occurs at the working path.
Generally, protection switching technology operates two ways, the first one is reverse mode (or revertive mode) that restores a state of switches that is connected to the working path after the fault is restored, and the other one is non-reverse mode (or non-revertive mode) that doesn't restore a state of the switches back to the working path after the fault is restored.
A fault of a path includes a signal failure (hereinafter, referred to as an ‘SF’) in which transfer of traffic is completely stopped and signal degrade (hereinafter, referred to as ‘SD’) in which traffic is partially transferred, but that causes a communication fault due to a bad transfer quality.
In this case, in order to detect SD, each node measures whether traffic is damaged in real-time, and when both a working path and a protection path are in an SD state, if the node uses a selector bridge, a traffic transmission path may be frequently changed. If a transmission path of traffic is frequently changed, traffic may not be appropriately transferred and thus in order to prevent this, each node of a network may use a permanent bridge or a broadcast bridge.
However, when the node uses the permanent bridge, traffic is always transmitted to the working path and the protection path and thus a network resource or a bandwidth of the path may be wasted. Particularly, in a packet-based network, there is a burden that should use a packet forwarding engine of a high performance for traffic copy.
Further, because the broadcast bridge copies traffic only when a fault occurs at a path, the broadcast bridge is more efficient than the permanent bridge, but when protection switching is performed in a non-reverse mode, a network resource may be still wasted. That is, even after a fault of the path is restored, when the broadcast bridge operates with non-reverse mode of operation method using both the working path and the protection path for traffic transmission, there is a problem that traffic is still copied and a resource of the network is wasted.